Did you put them outside? If you did te sun will get to all the branches and do its job.
Inside the actal depth of the light penatration depends on the light. The bigger and stronger the light, the taller your plants can be. If you stay with the flouro lights only the buds within 12" willbe OK. (I am guessing as I never have grown under flouros). Just cut the plants off low and let them get bushie. Read about LSt and scrog in here